Embodiments and aspects generally pertain to the field of laser apparatus and methods; more particularly to improved laser amplifier operation and, most particularly to improved large-aperture, high-power, solid state amplifier operation via the embodied apparatus and methods disclosed herein below.
High power (terawatt (TW) and petawatt (PW)) lasers are advantageous for applications in, e.g., medical physics (e.g., proton sources for cancer therapy); particle physics (e.g., particle accelerators); and nuclear physics (e.g., fusion, weapons, plasma and x-ray sources). It has been reported that current demand is for 100 mm diameter (Ti:sapphire) crystals, which is projected to grow up to 250 mm diameter crystals in a few years. A PW laser pulse of 30 fs duration requires a pulse energy of 30 J, necessitating 30 cm2 area of gain medium (e.g., Ti:sapphire). Beam clipping considerations thus dictate a Ti:sapphire crystal of the order of 10 cm diameter. Thus, large aperture Ti:sapphire amplifiers are a key requirement for the new PW class of ultrafast-ultrapowerful lasers.
Parasitic (or transverse) lasing in large-aperture, high-power, solid state amplifiers is a common problem that limits the final output power of the amplification process. Parasitic lasing occurs when the transverse gain exceeds the transverse losses caused by the Fresnel reflections at the edge of the gain medium, leading to transverse laser oscillations. For an untreated Ti:Sapphire crystal, for example, the Fresnel reflections are ˜7% for an incident angle normal to a surface exposed to air, and as such the threshold of parasitic lasing occurs when the transverse gain reaches 15. At the onset of parasitic lasing, the energy stored in the amplifier can be quickly drained, which has the effect of clamping the gain available to the intended longitudinal amplification process.
Traditional methods for reducing parasitic lasing in solid state lasers involve the use of absorbing thin films, optical coatings, and index-matching approaches to reduce the possibility that ASE photons reaching the laser gain medium boundaries are reflected back into the medium. This can be accomplished, e.g., by cladding selected boundaries of the gain medium with material that efficiently absorbs incident ASE photons.
TW/PW lasers also require thermal management. Heat can ruin the beam, lead to component damage, and limit the repetition rate, problems whose solutions include operating at very low repetition rates, water cooling, and cryogenic cooling.
A shortcoming in the known art recognized by the inventors is the incompatibility of the absorbing thin films, optical coatings, and index-matching approaches in high-vacuum and/or cryogenic environments, which are too extreme for these traditional solutions. Previous index matching methods for suppressing transverse/parasitic lasing cannot be used in cryogenic and/or high-vacuum environments; may not permit tuning of the index of refraction for exact index matching or for adaptability to wide ranges of gain media; may not support tuning of the absorption profile of the index matching region.
The inventors have therefore recognized the advantages and benefits of overcoming the aforementioned shortcomings and incompatibilities by providing solutions for reducing parasitic lasing in high power lasers operating in high-vacuum and/or cryogenic environments using an index-matching approach as disclosed by the embodiments herein. The recognized advantages and benefits enabled by the disclosed and claimed embodiments include, but are not limited to, laser systems operating in cryogenic and/or high-vacuum environments, where previously no other index matching solutions have proven viable; laser systems already using index-matching methods, to which the benefits of cryogenic cooling (e.g., improved beam quality, ability to operate at a higher repetition rate or average power, etc.) or vacuum compatibility can now be extended; laser systems where a cladding/coating material with a tunable index of refraction is advantageous (e.g., cases where: a very specific refractive index is required; the gain medium has an uncommon refractive index; many different gain media used in a system could be supported by the same technique; etc.); and laser systems where a cladding/coating material with a tunable absorption is advantageous (e.g., gain media where careful thermal management is important).
An exemplary commercial use of the embodied invention is in high-power, solid state laser amplifiers (Ti:Sapphire, Nd:YAG etc.). Some suppliers offer high-power laser amplifiers with an anti-transverse-lasing solution but without the benefits of cryogenic cooling, while other suppliers offer high-power laser amplifiers with cryogenic cooling but without the possibility of index matching. The embodied solutions could be used as part of an amplifier capable of offering the benefits of both cryogenic cooling and anti-transverse lasing. Another potential use could be in a laser system that could benefit from index matching if other index matching methods are not convenient or available.